Add trivial equivalence class support in tr(1) and update manpage

Equivalence classes are a hard matter and there's still no "standard"
way to solve the issue.
Previously, tr would just skip those classes, but it's much
better when it resolves a [=c=] to a normal c instead of treating
it as a literal.

Also, reflect recent changes in the manpage (octal escapes) and fix
the markup in some areas.

tr.1

@@ -25,22 +25,27 @@ Squeeze repeated characters matching
 .Ar set1
 or
 .Ar set2
-if -d is set.
+if
+.Fl d
+is set.
 .El
 .Sh SET
 .Bl -tag -width Ds
-.It Literal 'c'
-.It Escape sequence '\ec'
-\e\e, \ea, \eb, \ef, \en, \er, \et, \ev
-.It Range 'c-d'
-.It Repeat '[c*n]'
+.It Literal Sy c
+.It Escape sequence Sy \ec
+\e\e, \ea, \eb, \ef, \en, \er, \et, \ev, \eO[OO]
+.It Range Sy c-d
+.It Repeat Sy [c*n]
 Only in
 .Ar set2 .
 If n = 0 or left out, set n to length of
 .Ar set1 .
-.It Character class '[:class:]'
+.It Character class Sy [:class:]
 See
 .Xr wctype 3 .
+.It Equivalence class Sy [=c=]
+Resolve to
+.Sy c .
 .El
 .Sh TRANSLATION
 If no options are specified,

tr.c

@@ -143,6 +143,12 @@ nextbrack:
 
 			/* CLASSES [=EQUIV=] (skip) */
 			if (j - i > 3 && rstr[i + 1] == '=' && rstr[m - 1] == '=') {
+				if (j - i != 4)
+					goto literal;
+				(*set)[setranges].start = rstr[i + 2];
+				(*set)[setranges].end   = rstr[i + 2];
+				(*set)[setranges].quant = 1;
+				setranges++;
 				i = j;
 				continue;
 			}